Laser-Resonator
Dieser Versuch soll einen Einblick in die Arbeiten in einem Optiklabor geben. Der Umgang mit
typischen optischen Komponenten; Konzeption, Aufbau und Justage eines optischen Aufbaus sollen
vermittelt werden.
Im speziellen soll ein bereits vormontierter Titan:Sapphir-Laser justiert und somit der Laserbetrieb
ermöglicht werden. Mit Hilfe der Spektren sollten Charakteristika und Anwendungsgebiete der
verschiedenen Lasertypen erarbeitet werden.
Wichtig:
Die in diesem Versuch verwendeten Laser sind in die Laserschutzklasse 4 eingeordnet.
Die Strahlung dieser Laser kann zu schweren Verletzungen der Haut und vor allem der Augen
führen (vollständige Erblindung!).
Bereits die diffuse Strahlung ist gefährlich. Bei Laserbetrieb darf der Praktikumsraum
grundsätzlich nur mit geeigneten Schutzbrillen betreten werden.
Aufgabe 1:
Justieren Sie den Strahl des Helium-Neon-Lasers mit Hilfe zweier Spiegel durch zwei Blenden.
Aufgabe 2:
Installieren Sie den Einkoppler und verbinden sie diesen über die Glasfaser mit dem Spektrometer.
Justieren den Laserstrahl auf den Einkoppler und nehmen Sie das Spektrum des Laserlichts auf.
Aufgabe 3:
Bringen Sie, nachdem der Pumplaser eingeschaltet wurde (Betreuer), durch Justage der Spiegel den
Ti:Sa-Aufbau in den Laserbetrieb.
Aufgabe 4:
Messen Sie das Spektrum des Lasers. Wählen Sie durch Justage des Aufbaus verschiedene
Laserwellenlängen aus und regen Sie höhere transversale Moden an. Erläutern Sie den Zusammenhang
mit der Resonatorgeometrie.
Aufgabe 5:
Messen und erläutern Sie das Spektrum der Fluoreszenz des Ti:Sa-Kristalls.
Aufgabe 6:
Bestimmen Sie die Laser-Schwelle. Optimieren Sie mit Hilfe des Powermeters und der „beam-walk“Technik die Ausgangsleistung des Lasers. Bestimmen Sie die Laserschwelle abermals.
Aufgabe 7:
Diskutieren Sie auf Grundlage der Spektren Anwendungsmöglichkeiten eines Titan:Saphir-Lasers und
gehen Sie dabei insbesondere auf den Pulsbetrieb (Modelocking) ein.
Machen Sie sich bitte zur Vorbereitung mit folgenden Begriffen vertraut:
Laser (aktives Medium, Besetzungsinversion, Laserniveaus, Resonator, Laserschwelle...),
Laserschutz, dielektrischer Spiegel, Spektrometer, Glasfaser, Brewsterwinkel, Frequenzverdopplung,
Modelocking
Literatur z. B.:
Hecht, Optik
Demtröder, Laserspektroskopie
Saleh, Fundamentals of Photonics…
Setup and Alignment of a Titan:Sapphire
Laser
Dr. Christiane Becker, Dr. Nicolas Stenger, Justyna Gansel,
and Fabian Niesler
Karlsruher Institut fuer Technologie
October 28, 2010
1
Safety notes
During this experiment you will use two lasers: a pump laser Verdi V5 and a
Titan:Sapphire (Ti:Sapp) laser. The pump Laser can yield a maximal power
of 5 W which is classified as a class 4 laser. That means that direct exposition
of light can damage your skin when the beam is focused. Scattered light
from a reflecting surface can irremediably damage your eyes. That’s why it
is highly recommended to wear safety or alignment goggles. The Ti:Sapp
laser will still yield a power of 0.5 W which classifies it as a class 4 laser too.
It is even more trearterous than the pump laser. Actually the pump laser is
emitting in the green spectrum and hence can be easilly seen by your eyes.
The Ti:Sapp emits around λ = 800 nm, so that the laser beam will be hardly
seen by your eyes. The weak red light you will see is only the small seeable
part of the whole spectrum of the Ti:Sapp. This apparent weakness does
not represent the real intensity of the Ti:Sapp. Scattered light can destroy
irremediably the retina of your eyes.
On top of that the alignment goggles you will use to align the Ti:Sapp are
not protecting your eyes from the red part of the electromagnetic spectrum.
Otherwise you won’t be able to see the light going out of the output coupler
and hence won’t be able to see if the optical resonator is lasing or not.
So please be very careful, don’t look directly into the beam direction of both
lasers. Always wear safety or alignment goggles.
To avoid as much as possible scattered light from reflecting surfaces you
will be asked to get rid of all the jewelry you wear like finger rings, neck
laces, everything that can reflect light into your eyes. Please read carefully
the laser safety documents during the week before the experiment. Your
supervisor will ask you questions on what you’ve learn before you start the
experiment. Before you start the experiment your supervisor will give you
brief instructions on laser safety. Please follow carefully his instructions.
2
Basics of alignment
In this section the basics of optical alignment are described. These fundamental tools are useful for a broad range of optical setups: Laser resonators,
interferometers, autocorrelations, cross-correlations and optical imaging. As
our aim is the setup of a titan:sapphire laser we will focus on laser beam
alignment.
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Figure 1: Typical situation in a laser laboratory: A laser beam is directed
into a complicated optical setup, containing many mirrors, lenses, delay lines,
prisms,... In case (a) any (absolutely possible) deviation of the initial direction of the laser beam would result in a complete re-alignment of the optical
setup. In the situation of (b) the beam path is defined by two apertures
(A1, A2) in front of the setup. Only adjustments of mirrors M1 and M2 are
necessary.
2.1
Alignment of a beam through two apertures with
two mirrors
We consider the frequent case where a laser beam is directed into a very
complicated optical setup consisting of many mirrors, lenses, delay lines and
prisms (see Fig. 1). If the beam is steered directly into the optical setup
any - absolutely possible - deviation of the beam direction would cause an
annoying re-alignment of the optical setup (a). Directing the beam through
two apertures with two mirrors however, as seen in part (b), has many advantages: The course of the laser beam is completely defined by the two
apertures. Hence, the first step before building up a complicated optical
setup is defining the beam path by two apertures in front. Any directional
deviation of the laser beam can be corrected by aligning the beam with the
two mirrors through the two apertures again. It is even very easy to change
the laser source as seen in grey in Fig. 1 (b). You just need two additional
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mirrors.
Initial alignment procedure
As a general rule, beams should be parallel to the surface of the optical table.
A change of the beam hight should be avoided if ever possible. Furthermore,
deflections by the mirrors by 90 ◦ are preferred. This is not only important
for the sake of clarity and hence for safety, but also to keep track of the
polarization direction of light.
As another general rule, for first alignment choose a weak (< 1 mW) and
visible laser beam. A good choice is for example a Helium-Neon laser with
a wavelength of λ = 633 nm. But be careful and make sure that the laser
power is attenuated below 1 mW with some grey filters! If you want to use
another (stronger) laser source at the end, a configuration as shown in Fig. 1
(b) is meaningful: ”Laser 1” is the strong laser, ”laser 2” the alignment laser.
The second mirror of ”laser 2” should be a kinematic mirror so that it can
be removed if ”laser 1” is used.
Placing the mirrors/rough alignment:
1. As beam height we choose the height of the exit of the strong laser.
Mount the week laser in such a way that its exit has the same hight.
2. Place mirror M1’ into the beam path of the week laser as sketched in
Fig. 1 (b). The beam should be centered in the mirror surface.
3. Place mirror M2’ into the beam path of the week laser as sketched in
Fig. 1 (b).It should be a kinematic (removable) mirror. Take care that
the beam hits the mirrors in the center.
4. Adjust the position and orientation of mirror M2’ such that the reflected beam is roughly above a row of screw holes.
Vertical alignment/definition of the beam height:
1. Take a 90 ◦ alignment tool and mark it at the desired height with a
pencil.
2. Place it in direction of the beam, for higher accuracy as far away as
possible.(Limitations: The length of your arm or available space on
optical table.)
3. Adjust the beam vertically with the second mirror M2’ onto the marking.
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4. Place the 90 ◦ alignment tool into the beam right after mirror M2’.
5. Adjust the beam vertically with the first mirror M1’ onto the marking.
6. Repeat the steps 2 to 5 iteratively unless the beam is parallel to the
surface of the optical table at the desired height.
Horizontal alignment above a row of screw holes:
1. Release carefully the screws that were fixing mirror M2’ on the optical
table.
2. Place the 90 ◦ alignment tool a few centimeters after mirror M2’ such
that the edge is at the middle of a screw hole.
3. Adjust the position (not the orientation!)of M2’ along the axis M1’M2’ such that beam hits the edge. Moving mirror M2’ along this axis
ensures that the beam stays centered on its surface.
4. Place the alignment tool as far away as possible in direction of the
beam. The edge should be at the middle of a screw hole in the same
row as in step 2.
5. Adjust the orientation (not the position!) of M2’ horizontally such that
beam hits the edge.
6. Repeat the steps 2 to 5 iteratively unless the beam is aligned above the
row of screw holes.
7. Tighten the screws of mirror M2’ for fixing it on the optical table.
Now we can be sure that our beam is aligned parallel to the table surface at
our desired height and centered above a row of screw holes. This beam path
must be fixed by two apertures. The first should be fixed close to mirror
M2’, the second as far as possible in direction of the beam. For exercise we
can check before if the apertures have exactly the same height.
Adjusting two apertures to exactly the same height:
1. Place one aperture into the beam at a defined position. It is not necessary to fixe it on the table.
2. Adjust the height of the aperture such that the beam fits the maximally
closed hole.
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Figure 2: Alignment procedures for defining a beam path with two apertures and two mirrors (a) or in the rare case with only one effective mirror
(b). Case (a) has to be applied more often, e.g., in ”‘everyday alignment”’.
Case (b) is important for initial alignment above a row of screw holes while
ensuring the beam centered on the mirror surface simultaneously.
3. Remove the first aperture and place the second aperture into the beam
at the same position.
4. Adjust the height of the aperture such that the beam fits the maximally
closed hole.
The two apertures can be screwed in the optical setup now and define the
beam path completely.
Everyday alignment procedure
If the beam path is once defined by two apertures, the everyday alignment
procedure is quite easy. Even if the laser decides every day to change its
direction slightly, it does not matter.
Alignment of a beam through two apertures with two mirrors:
1. Close the second aperture A2 as much as possible.
2. Center the beam on the hole of A2 by adjusting mirror M2’ vertically
and horizontally.
3. Close the first aperture A1 as much as possible.
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Figure 3: Alignment card with a hole and a slit. The incoming beam (solid
line) passes the hole/slit and is reflected back by a mirror (dashed line). The
spot of the reflected beam should be somewhere near the hole/slit and can
be directed onto the incoming beam by adjusting the mirror vertically and
horizontally.
4. Center the beam on the hole of A1 by adjusting mirror M1’ vertically
and horizontally. Open A1 again afterward.
5. Repeat steps 2 to 4 iteratively unless the beam is perfectly aligned
through the two apertures.
2.2
Aligning a back-reflected beam exactly above the
incoming beam.
In order to build a laser you necessarily need an optical resonator. Therefore
the end mirrors of the resonator have to be aligned such that the beam
impinging onto the end mirror and the reflected beam lay exactly above each
other.
1. Take a white card. Drill an small hole into it or cut a narrow slit at
the edge (see Fig. 3).
2. Center the beam on a mirror perpendicular to the surface.
3. Hold the white card into the beam such that the beam can pass through
the hole (slit). The reflected beam should appear somewhere in the
vicinity of the hole (slit). If you can’t find it put the card closer to the
mirror.
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4. Adjust the mirror horizontally and vertically that the reflected beam
passes through the hole (slit) backwards.
2.3
Beamwalk
On a later stage of the setup and alignment of the laser it will be necessary
to perform a so called ”beam walk”. The optical resonator might be aligned
perfectly but the crystal is hit on a bad area. Hence the beam must ”walk”
to another spot of the crystal.
1. Monitor the laser power with a power meter. Put your hands on the
two horizontal adjustment screws of the two end mirrors of the optical
resonator.
2. Adjust the first of the screws for maximal power, remember the value
and tilt it a little further in the same direction (power should go down
a little).
3. Adjust the second screw for maximal power. If the power is higher
than in step 2 adjust it a little further in the same direction.
4. Repeat steps 2 and 3 until the maximum possible power is achieved.
Try the beam walk in both directions.
5. Repeat the same procedure using the vertical alignment screws.
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2.4
Exercises
• Exercise 1
Align the beam of the helium-neon laser above a row of screw holes
at a height of 10 cm. Use a kinematic mirror as second mirror so that
eventually a second laser can be fiddled in afterwards. Apply the alignment schemes described in section ”Initial alignment procedure”. If
you are content with the result, fixe the beam path by screwing in two
apertures.
• Exercise 2
Practise the ”everyday alignment procedure” with two mirrors and two
apertures a few times.
• Exercise 3
Project the HeNe laser beam on a screen composedv by a beam blocker
and a piece of paper attached on it’s surface. Put the end of the
multimode fiber next to the screen and measure the spectre of the
HeNe.
What can you say about the shape of the HeNe spectrum?
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3
Setup and alignment of a titan:sapphire laser
The Verdi laser be on ”‘standby mode”’ your supervisor will turn it on for
you. Don’t do it alone.
This section is a ”‘alignment manual”’ for a titan:sapphire laser in continuous wave (cw) operation. All you need is an energy pump - in our case
a doubled Nd:Yag laser at λ = 532 nm - (What means that the Nd:Yag is
doubled?), a titan:sapphire-crystal, a chiller and water cables for cooling, and
other optical components.
Preliminary considerations
The resonator of a titan:sapphire laser usually has the so called ”Z-configuration”. Compare figure 2 at the end of this manual with the laser setup in
front of you and identify the different components.
Explain in a few words the role of each component.
Mounting and cleaning the optics
If the dielectric mirrors are polluted with dust try to remove it with a small
blow tool. Remaining dirt has to be removed with lens cleaning paper. Please
ask your supervisor to clean them for you. Don’t try by yourself these components are very sensitive!
Defining the beam path by two apertures
Use the Helium-Neon laser and align it with two mirrors above a row of screw
holes (see section 2.1). The second mirror should be a kinematic one. Fixe
the beam path by two apertures. Take care that there remains enough space
for two more mirrors that should deflect the strong pump laser through the
same apertures.
Initial alignment of the titan:sapphire laser
Don’t touch the center of the cavity and the Ti:Sapphire crystal!
this parts are very sensitive and expensive!
For the initial alignment the Helium-Neon laser should be used. This not
only minimizes the risk of dangerous reflected and scattered light but also is
more comfortable because no goggles have to be worn.
The beam passes through the focussing lens and the first concave mirror
in the center, propagates through the titan:sapphire crystal right in the middle and hits the second concave mirror in the center. (Why is the normal
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to the surface of the Ti:Sapp crystal oriented with a high angle according to
the impiging ray of the pump laser and why do we have inserted a polarizer
before the rasonator of the laser? Hint: it minimizes backreflection on the
surface on the surface) The resonator is in the symmetrical confocal configuration which ensures a tight focus of the 800 nm mode.
• What is a symmetric confocal configuration? Explain it in a few words.
Why do we use this configuration for the cavity? Discuss the stability
and losses of this resonator geometry with other kind of configurations
like the symmetric concentric one. Don’t enter too much into mathematical details (For more information look in Optik E.Hecht p.950
or in Fundamentals of photonics B.E.A Saleh p.310). The intensity
profile of the laser beam is directly depending on the geometry of the
resonator. What kind of beam intensity profile would you expect for a
laser composed of two symmetric concave mirrors?
What is the best distance between mirrors and crystal? (Don’t enter in
too much mathematical details, use the notions of optics you’ve learned
in lectures hints: light should make roundtrips in the cavity).
Check now again that the beam is centered on both spherical mirrors and
passes the crystal without hitting the edges. Do it carefully.
As the laser-resonator mirrors are optimized for λ = 800 nm and the
alignment laser has a wavelength of λ = 633 nm, the reflected beams are very
weak. In this case remove some of the grey filters in front of the Helium-Neon
laser for stronger signals.
The beam reflected from the end mirror should be depleted by the first
concave mirror, pass the crystal, hit the second concave mirror at the center
and be reflected again by the output coupler. This reflected beam has to be
centered onto the end mirror of the resonator. Align the beam reflected from
end mirror exactly above the incoming beam as described in section 2.2. Do
exactly the same for the output coupler.
Investigate the 360 ◦ surrounding of the optical setup for unwanted reflections. Block all of them with a beam blocker. Even if the reflections are quite
weak at the moment be aware that these reflections can be very dangerous
when the strong pump laser is used. For the undeflected pump beam use a
special beam dump specified for blocking high laser powers.
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Figure 4: Cooling system for pump laser and titan:sapphire crystal. The
temperature should be in the range of 18-20 ◦ C.
Cooling of the titan:sapphire crystal
Before switching on the pump laser the cooling system for the titan:sapphire
crystal has to be turned on. Check that the chiller is filled with enough clear
(destilled) water. One chiller is sufficient for cooling both, the pump laser
and the crystal. .
Final alignment
Don’t start this section without speaking with your supervisor!
The pump laser Verdi V5 is a class 4 laser even scatterd or reflected light
can damage irremediably your eyes. That’s whay it is strongly recommended
to wear safety or alignment goggles! Your supervisor will help you in these
part, please don’t do anything before talking to your supervisor.
Don’t insert fingers or paper between the focusing lens and the
second concave mirror
The power is there sufficient to burn skin and paper!
Make sure that there is a beam blocker after the output coupler,
insert the grey filter between the output coupler and the beam blocker
The light going out of the output coupler is very treaterous! The
main peak going out of the Ti:Sapp is around λ = 800 nm, so you probably
won’t see it. What you will see is only a small part in the spectrum around
λ = 700 nm. You might think that the intensity is not high but in fact the
power out of the laser is around 0.5 W. It is classified as a class 4 laser. Even
diffused light can damage seriously your eyes. On top of that the alignment
13
goggles are only protecting your eyes around λ = 532 nm. There is no protection around λ = 800 nm!
Get on the alignment goggles. Your supervisor will switch on the pump
laser. The power (power mode) is now set to the smallest possible value
(P=0.1 W). The beam should be visible inside the crystal via red fluorescence. Check that the beam passes the crystal without inner reflections at
the borders. For good laser performance it is also important that the losses
in the optical resonator are minimized. Place a white card behind the output
coupling mirror. Increase the laser power to 2-3 W (As usual be carefull to
scattered and reflected light). On the white card a large and strong and
red point should appear, effected by the fluorescence inside the crystal. You
might also see next to the large spot a smaller and weaker one. The stronger
point comes from the first passing through the crystal, the weaker is the
fluorecsnce of the Ti:Sapp crystal backreflected by the end mirror. Align the
end mirror to move the weak spot onto the strong spot unless both points
overlap. Do the same with the output coupler. Now the laser should start
lasing.
• By slightly moving the output coupler verticaly or horizontaly, you
should see appearing the different modes of the resonator (see page 953
in Optik form E.Hecht). Can you explain in a few sentences why you
observe different mode?
Now align the ouput coupler to obtain the TE00
Shut the beam of the Verdi with a beam blocker as explained by your supervisor. Close the shutter of the pump laser. Leave the room with the
alignment goggles on your nose and take the security goggles with you. Exchange the alignment goggles by the security goggles. The security goggles
are protecting your eyes against the green light coming out of the pump laser
and the red light coming out of the Ti:Sapp. During this time your supervisor will install the powermeter for the rest of the experiment. (Don’t do it
by yourself!) When everything is ready enter to the laser room and proceed
with the rest of the experiment.
Optimization
Once the laser is lasing we can optimize the alignment for maximum power.
Make a beam walk as described in section 2.3 with the two end mirrors of the
optical resonator. With an output coupler of 97% reflectance and an input
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power of 2.8 W (@ 532 nm) it is possible to obtain 0.4 W output power (@
800 nm). The supervisor will show you how to get more power by adjusting
the center of the cavity. Don’t do it by yourself !
Measurement of the Ti:Sapp spectrum
Place the end of the multimode fiber next to the Ti:Sapp crystal (don’t touch
it! it’s very sensitive). Measure the spectrum of the red fluorescence coming
out of the Ti:Sapp crystal. Insert a beam blocker with a piece of paper
glued on it between the grey filter and the powermeter. Take the end of the
multimode fiber connected to the spectrometer close to the beam blocker.
The scattered light intensity must be sufficient to measure the spectrum of
the Ti:Sapp.
• Compare the spectra from the HeNe, the fluorescence of the Ti:Sapp
and from the lasing spot of the Ti:Sapp. Why is the fluorescence and
the lasing spectra of the Ti:Sapp different?
Why are the spectra of the HeNe and the Ti:Sapp different.
• By slightly moving one degree of liberty of the output coupler, one can
change the lasing mode of the Ti:Sapphire laser. Take two or three
spectra with different modes (frequencies). Determine the frequency
difference between these modes and compare it with the actual length
of the resonator. What can you deduce from this measurement?
• Ti:Sapp lasers are often used in ultra short pulses experiments. To
obtain ultra short light pulses we use a technique called ”‘mode locking”’technique.
Explain in a few sentences what is the mode locking technique.
Explain why the Ti:Sapp laser is appropriate for producing ultra short
pulses by analyzing its spectrum and by comparing it by what you’ve
learned on the mode locking technique.
Additional material to read:
• Eugene Hecht, Optik (4.Auflage), Oldenburg (2005). See Chapter 13,
p. 941 to p. 960.
• Bahaa E. A. Saleh, Malvin Carl Teich, Fundamentals of Photonics,
Wiley (2007). See Chapters 10, 14, and 15.
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